1. Field of the Invention
The present invention relates generally to line pressure control arrangements for automatic automotive transmissions and more specifically to a line pressure control arrangement which is responsive to the temperature of the oil being used to engage the friction elements.
2. Description of the Prior Art
JP-A-62-62047 discloses a transmission control arrangement wherein the level of line pressure which is used to engage and disengage the various friction elements of the transmission is varied in accordance with the load on the engine as indicated by the position of the throttle valve.
Further, in order to reduce friction element engagement shock the above arrangement is such as to increase the pressure slowly during the initial stage of the engagement and thus permit an amount of slip to occur.
However, the lubricative and viscosity properties of the oil used to engage the friction elements reduces with increase in temperature. Accordingly, when the temperature of the oil rises even though the pressure level is being adjusted in accordance with the engine load, a problem occurs that the reduced lubricative and viscosity properties of the oil tends to permit excessive slippage between the elements of the friction element or elements which are being engaged.
This of course leads to increased wear rate and tends to shorten the working life of the friction element in addition to inducing an undesirable change in the shifting characteristics of the transmission.